The Cherbourg roadstead has been radically altered by the installation of industrial infrastructures (major dredging and reclamation works in 2015 and 2016 to allow the passage of deeper-draught ships and the creation of a new quay: 39 ha). A research program was carried out before and after the transformation of the port platforms to improve knowledge of the roadstead and assess its impact. Three complementary environmental approaches were used: (1) morphological changes based on a comparison of bathymetric data; (2) changes in sediment cover based on granulometric data; and (3) the impact on macrofauna benthic communities. It appears that the new construction is the cause of a morphological change in the seabed, leading to the creation of multi-metric "hollows" and "humps", responsible for a general change in hydrodynamics and widespread silting of the Grand Rade (+14% silt). To the east, more than 10% of the well-sorted, unimodal sandy seabed has silted up after the extension. This siltation reduced the exceptional diversity of benthic habitats and has led to a drastic reduction in fragile species (amphipods) and an increase in opportunistic species, probably altering the entire structure of the food web.